In the information recording field, developmental work related to optical discs is proceeding. Optical discs are widely employed in tasks ranging from industrial use to consumer use because recording and reproduction can take place without contact, large files can be stored at a low cost, and there are read-only, write-once, and rewritable optical discs for differing applications.
The capacities of the various optical disc types mentioned above have been increased by decreasing the size of the pits and marks etc. representing data written on the recording tracks on the information recording surface of the optical disc, by using semiconductor laser beams with shorter wavelengths for recording and reproducing, and by using objective lenses with larger numerical apertures, thereby reducing the focused spot size on the focal plane.
For example, a 650 MB capacity was achieved in a CD (compact disc) with a 1.6-μm track pitch by using an infrared laser beam with a wavelength of substantially 0.78 μm from a semiconductor laser, and an objective lens with a 0.45 numerical aperture (NA), for recording and reproducing data on the recording tracks.
Next, a 4.7 GB capacity was achieved in a DVD (digital versatile disc) with a 0.74-μm track pitch by using a red laser beam with a wavelength of substantially 0.65 μm from a semiconductor laser and a 0.6 NA objective lens for recording and reproducing data on the recording tracks.
A 25 GB capacity was achieved in a BD (Blu-ray disc) with a 0.32-μm track pitch by using a blue laser beam with a wavelength of substantially 0.405 μm from a semiconductor laser and a 0.85 NA objective lens for recording and reproducing data on the recording tracks.
Through CDs, DVDs, and BDs, optical discs have evolved toward larger capacities as described above. Upward compatibility is generally desired in optical disc equipment; at present, optical disc devices are being sold that can record and reproduce on discs of all three types: CD, DVD, and BD.